1. Field of the Invention
The present invention relates to a ferroelectric element and an actuator using the same, and an ink jet head and an ink jet recording device.
2. Discussion of the Related Art
An ink jet head of an ink jet recording device includes a pressure generating part having a pressure generating chamber containing ink liquid, and an actuator part for driving and causing the pressure generating chamber to eject ink drops. A ferroelectric element includes a ferroelectric film, and an individual electrode and a common electrode. Voltage is applied to between those electrodes and hence, to the ferroelectric film, so that the ferroelectric film shrinks and expands by its piezoelectric effect. The shrinking/expanding of the ferroelectric film vibrates a vibration plate, which is located adjacent to the ferroelectric element. The vibrating vibration plate drives the pressure generating chamber associated therewith to cause it to eject ink drops through a nozzle orifice of the pressure generating chamber.
In the ferroelectric element thus structured, the ferroelectric film including the upper electrode are barely formed, and the ferroelectric element is easily affected by ambient actors, e.g., humidity. If the ferroelectric element suffers from crystal structures defects, a called water path grows starting from such a defect. The breakdown voltage characteristic of the ferroelectric element is deteriorated, and in an extreme case, dielectric breakdown occurs in the ferroelectric element.
Further, in informing the ferroelectric film of the ferroelectric element, foreign materials possibly enters the ferroelectric film under forming. When the foreign materials enters the ferroelectric film, crystal structure defects and crystal grains abnormally grow in the film. As a result, defects, such as gaps and pin holes, are formed in the boundary between the ferroelectric film 10a and abnormally grown grains. In a ferroelectric element formed using such a defects-contained ferroelectric film, dielectric breakdown sometimes occurs in the ferroelectric film under a voltage applied to between the individual electrode and the common electrode, the voltage being lower than a breakdown voltage proper to the film. If water enters the defects, the insulating property of the film is considerably deteriorated. Gaps on grain boundaries and pin holes are present between the upper and lower electrodes. When water enters there, oxide insulating materials are deoxidized by an electrochemical reaction, and metals separate out, and the ferroelectric film is dielectrically broken for a short time.
The techniques as given below are proposed to ameliorate the insulating property of the ferroelectric film, and to prevent the dielectric breakdown of the film.
In Japanese Patent Publication Hei-6-112545, the whole upper and lower surfaces of a functional layer constituted by alternately depositing piezoelectric ceramic layers and internal electrodes are coated with protective layers formed of a piezoelectric ceramic or insulator.
In Japanese Patent Publication Hei-2-240976, there is disclosed a technique to prevent the decrease in insulation resistance between inner electrodes in use for a long time under the environment of intrusion and permeation of moisture and high humidity by providing a petroleum wax layer which is formed at least on side surface of a laminated type piezoelectric actuator
In the technique of Japanese Patent Laid-Open No. 6-112545, the side surfaces remains exposed. In the technique of Japanese Patent Publication Hei-2-240976, the upper surface remains exposed. Therefore, the piezoelectric actuator is affected by environmental conditions, through the side surfaces or the upper surface. The actuator still suffers from the problem stated above.
In the technique of Japanese Patent Publication Hei-6-112545, the piezoelectric actuator which outputs longitudinal displacement by utilizing the lateral strains in the actuator, is structured so that lateral strains, parallel to the electrodes, are not produced in a functional layer. Since it is formed with a bulk, piezoelectric ceramic layer, its displacement quantity is large. Therefore, the restriction on the displacement of the piezoelectric ceramic layer by the protection film is relatively small. However, in a case where the ferroelectric film as a functional layer is thin and hence, its displacement quantity is small, a protecting film which is formed on the ferroelectric film restricts the displacement of the ferroelectric film, and the ferroelectric element sometimes fails to exhibit the element characteristic as intended. The problem of restricting the displacement of the ferroelectric film by the protection film is also present in the technique of Japanese Patent Publication Hei-2-240976.
Further, Japanese Patent Publication Hei-2-49471 discloses a semiconductor element having a lamination structure having a lower electrode, a polysilicon oxide film, a ferroelectric film, and an upper electrode, which are laminated in this order with the lower electrode as the structure bottom
Japanese Patent No. 3111416 (Japanese Patent Publication Hei-4-92469) discloses a technique to form an insulating film containing SiN as a main component between an active element, e.g., transistor, formed on the semiconductor substrate, and a capacitor formed of a ferroelectric material.
Japanese Patent No 3139491 (Japanese Patent Publication 2000-4004) discloses a ferroelectric element having a structure in which a ferroelectric material is sandwiched between two electrodes, and a normal dielectric layer formed with an SiO2 film, which is formed between a peripheral part of one of the electrodes and the ferroelectric film.
Japanese Patent Publication Hei-10-217458 discloses the following technique: In a piezoelectric element having a lower electrode, a piezoelectric film formed on the lower electrode, and an upper electrode formed on the piezoelectric film, a low-dielectric substance whose relative dielectric constant is lower than that of the piezoelectric film is formed at least in the grain-boundaries exposed region o the piezoelectric film.
In those conventional techniques, the breakdown voltage of the ferroelectric film is increased by laminating the polysilicon oxide film, SiN film or the like over the ferroelectric film. However, the adhesion property between any of those layers and the ferroelectric film is poor, and device reliability is not satisfactory.
The relative dielectric constant of the polysilicon oxide layer, the SiN film or the like is 10 or smaller, much smaller than 100 to 1000 of the relative dielectric constant of the ferroelectric film. For this reason, if the film thickness is increased to secure its satisfactory breakdown voltage, the voltage acting on the ferroelectric film becomes low. As a result, the drive voltage of the actuator must be increased.
Problems that arise when the ferroelectric film is used in combination with the polysilicon oxide film and the SiN film are due to the fact that the physical properties of those films, such as compositions, crystal structures, thermal expansion coefficients, are greatly different from those of the ferroelectric film. To cope with this, it is necessary to increase a thickness of the film in order to cover the gaps between the grains and pin holes are present between the upper and lower electrodes.
The publication of Japanese Patent Publication Hei-10-217458 describes that since the insulating property of the piezoelectric film is low in the crystal grain boundaries, the substance of low relative dielectric constant is formed in a region of the film where the crystal grain boundaries are exposed, whereby a voltage to be applied to the crystal grain boundaries is lowered to thereby improve the breakdown voltage characteristic of the piezoelectric film. The technique is applied to the piezoelectric film of the type in which adjacent crystal grains are continuously bonded and hence, there is no three dimensional spatial defects. Therefore, the technique is not effective for preventing the dielectric breakdown of the ferroelectric film suffering from such detects as gaps between the crystal grains and pin holes, to which the solution by the present invention is directed, viz., for the dielectric breakdown of the film, which is due to those defects.
The conventional technique advantageously improves the dielectric strength of the actuator, but involves the following problems: its poor adhesion property leads to the lowering of electrical and mechanical strength reliabilities, and the formation of the low relative dielectric constant layer needs a high drive voltage.